Donegal Bay
Cuan Dhún na nGall

Overview

Donegal Bay is situated in the northwest of Ireland, with a large portion of its northern and eastern coast falling within Co. Donegal county boundaries. The remaining portion of Donegal Bay's coast is in north Co. Sligo. The bay can be defined as an area that extends from Rathlin O’ Birne Island to the north, to the east as far as Donegal town and southwest out to Cooladoon/Dooneragh point.

Location of Donegal Bay in northwest Ireland.

Donegal Bay is approximately 36 km wide from Rathlin O’ Birne Island to Cooladoon/Dooneragh point and is close to 46 km long from Donegal town to the mouth of the bay. Within the bay, a series of headlands and cliffs are the most striking physical landforms with the Slieve League sea cliffs, St. John’s Point and Doorin Point on the north coast and Kildoney Point, Mullaghmore Head and Streedagh Point along the south coast.

The small island of Inishmurray, located around 5.5 km northwest of Streedagh Point, is famous for the Christian monastery founded there by St. Molaise in the 6 th century. The spectacular sea cliffs at Slieve League fall 595 metres to sea level at their highest point and are the second highest sea cliffs in Ireland.

Photo of the sea cliff at Slieve League taken from the Celtic Voyager during the CV08_01 leg of survey work in early 2008. (Click on image for more detail)

INFOMAR Survey History

INFOMAR surveys of Donegal Bay date back to INSS times when Killybegs, Co. Donegal was often used as the port where research vessels sailed from or returned after surveying the deep waters of the Rockall.

Sunset in Donegal Bay through the A frame taken during the survey by the Celtic Voyager in early 2008. (Click image for more detail)

In 2002 the Celtic Voyager surveyed a large portion of the deeper waters of outer Donegal Bay as part of a larger survey area extending west to Erris Head, Co. Mayo. In early 2008, the Celtic Voyager was again used to survey the areas close to the coast all around Donegal Bay.

Later in the summer of 2008, BLOM Aerofilms flew a lidar survey over the inner bay close to Donegal town, capturing some impressive datasets of the Murvagh sand spit.

The RV Keary and Cosantóir Bradán mapped the shallower waters along the coast of Donegal Bay, in 2014.

Coverage from survey legs of Donegal Bay. (Click on image for more detailed map).

Shaded Relief

The 3D appearance is achieved using software called Fledermaus. By using some vertical exaggeration, artificial sun-shading (usually as if there is a light source in the nw 315°) and colouring the depths using various colour maps, it is possible to highlight the subtle relief of the seabed. This helps us to quickly understand the variation in depths.

MBES image showing an overview of the coverage that has been achieved in Donegal Bay by the INFOMAR project. Download the Donegal Bay charts in .pdf here

MBES image showing a section of coastline from Slieve League cliffs in the west to St. John's Point in the east. The image also shows an area of rock outcrop south west of Carrigan Head known as Teelin Knowl. Closer inspection of the seabed around St. John's Point shows impressive outcrops of rock.(Click on image for more detailed map)

MBES image showing a large ridge running roughly north to south offshore from Rossan Point, Co. Donegal. This is a glacial deposition feature called a moraine. Moraines are composed of sediments which were eroded and transported by a large ice sheet from the Northwest Ice Dome. After the Last Glacial Maximum (~14,000 years ago) there was a general trend of retreat of ice. Evidence of this is seen in the lower ridge features in the top right corner of the image above, showing the episodic retreat of the ice sheet depositing glacial sediments as it went. However, at some stage it is thought that the ice retreat was slowed/stopped or readvanced allowing large amounts of sediment to be deposited in one ridge that eventually formed this spectacular moraine feature, now lying in 60 metres of water. Research has shown this feature can be traced on land.(Click on image for more detailed map)

MBES image showing a stretch of north Co. Sligo coastline in southern Donegal Bay from Mullaghmore Head to Streedagh Point. The most notable feature in the image is the dramatic ledge seen south west of Streedagh Point where the rock outcrop drops off at a near vertical angle. Closer inspection of the data suggests that this the ledge area is the same gently dipping, thinly bedded limestones of Streedagh Point which are widely known for their abundant fossil assemblages. The image also shows a large rock outcrop on the seabed west of Mullaghmore Head which can be traced south west to the island of Inishmurray, famous for its monastery mentioned above. (Click on image for more detailed map)

Backscatter

Multibeam Systems also collect additional information, including the strength of the acoustic signal (or return) from the seafloor. This is known as Backscatter. Differing seafloor types, such as mud, sand, gravel and rock will have different Backscatter values depending on the amount of energy they return to the sonar head. Rocky areas will typically have high returns while soft sediments like mud are more likely to absorb energy and have low Backscatter returns. These differing values are used to generate a grey-order image (i.e. dark for high returns, bright for low returns) of the seabed which can be used to examine the nature of the seafloor.

MBES backscatter overview image of Donegal Bay. Dark areas near the coast and around Inishmurray correspond to rock outcrops and coarser sediments. The light grey areas correspond with medium to fine grained sediments such as sand and mud. Download the Donegal Bay charts in .pdf here

Ground Truthing

A range of seabed sampling has been undertaken in Donegal Bay, both historically and under the INFOMAR project. The locations of the grab samples have been mainly determined by the seabed classification made from the multibeam data.

Complete map of sampling locations of the northwest coast of Ireland including Donegal Bay. (Click image for more detailed map).

Seabed samples were taken in 2002 by the RV Celtic Voyager in the the deeper waters of outer Donegal Bay (blue circles).

From the 6th to 8th of July 2009, a ground truthing or seabed sampling survey was undertaken in the region of Donegal and Sligo Bay. 80 samples were collected using a day grab (pink circles).

Seabed Classification

The classification of the multibeam dataset from Sligo and Donegal Bays resulted in the creation of a 6 class classification divided into two types of rock, reflecting the different textures observed from rock outcrops in the bay. Three more classes divided the sediments into Fine Sand to Mud, sand and a sand veneer over rock. The final class displays areas of soft ground but showing high backscatter, possibly driven by biogenic content.

Seabed Habitats \ Seabed Substrate

The seabed substrate information has been derived from a combination of the analysis of geophysical data (multibeam echosounder bathymetry and backscatter) and groundtruthing data acquired as part of the INSS and INFOMAR seabed mapping programmes.

Geology

Donegal Bay Geology

Donegal Bay sees a transition along its coastline from predominantly limestones, sandstones, mudstones and shales of the Carboniferous period in the south, east and northeast to the much older, heavily deformed rocks of the Dalradian group in the northwest (schist) – these are metamorphic rocks, which hundreds of millions of years ago were sedimentary rocks before being caught up in intense tectonic upheavals.

The younger limestones can be distinguished in the seabed data by their more regular, layered appearance.